Many neurological diseases are due to the degeneration of specific populations of neurons. An important contribution of basic research towards the understanding of such disorders would be the understanding of how postmitotic neuron survival and function is maintained in the normal organism. A prominent hypothesis is that adult neuron survival and function is dependent upon the availability of trophic factors produced by target tissues. The long term goal of this project is to identify and isolate such growth factors, and to determine the mechanisms by which they regulate neural development and maintenance. Our model system is the innervation of ocular tissues by neurons of the ciliary ganglion, thus this work also has application to the further understanding of how innervation of the eye is maintained by growth factors produced in the choroid layer, pigmented epithelium, iris, ciliary body and neural retina. We have purified, cloned and expressed a factor that supports the survival of chick ciliary ganglion (CG) neurons called growth promoting activity (GPA). Activities in eye extract and conditioned medium from choroid layer cells that control cholinergic and peptidergic transmitter phenotype expression in CG neurons termed ChAT stimulating activity (CSA) and somatostatin stimulating activity (SSA) have also been identified. Preliminary data indicates that SSA is activin. The primary goal of this application is to test the hypothesis that GPA, CSA, and SSA are all target-derived molecules that regulate the development of CG neurons. In order to establish that a molecule is a target-derived trophic molecule a number of criteria must be fulfilled. Thus, the specific aims of this proposal are: 1) To test whether the temporal and tissue specific pattern of the expression of GPA and GPA mRNA is consistent with that of a target derived trophic factor; 2) To use immunoprecipitation and HPLC to test whether GPA is secreted by a target tissue of CG neurons, and if GPA is secreted to determine the mechanism of secretion; 3) To test whether addition of GPA to developing chicken embryos is able to rescue CG neurons from dying and to test whether antibodies directed against GPA are able to increase the number of neurons dying; 4) To use iodinated recombinant GPA to test whether a saturable high affinity binding site for GPA is found on CG neurons; 5) To isolate and clone SSA and CSA, and to produce specific antibodies in order to test whether SSA and CSA are target derived factors that influence transmitter status of CG neurons.